Reflecting mirror antenna unit with diverse curvature

ABSTRACT

In a reflecting mirror antenna unit, the surface of the reflecting mirror is divided into a plurality of parts, and the parts are made different in thickness from one another, for instance, by bonding metal tapes different in thickness thereto, to change the phase distribution on the reflecting mirror, whereby the configuration of the radiation pattern due to the current distribution on the reflecting mirror is readily changed.

BACKGROUND OF THE INVENTION

The present invention relates to a reflecting mirror antenna unit, andis intended to provide a reflecting mirror antenna unit in which aradiation pattern due to the current distribution on the reflectingmirror can be varied readily.

With a conventional antenna unit of the type in which a radio waveradiated by a primary radiator thereof is reflected by a reflectingmirror, it is disadvantageous in that, even if the antenna unit is sodesigned that for instance a side lobe is low, the side lobe is higherthan a desired one for instance because of errors involved inmanufacturing the mirror surface. Among these side lobes, one formed ina direction near the bore-sighting axis is often due to the variation ofthe current distribution which is caused by deformation of the mirrorsurface. It is difficult to decrease this side lobe.

Heretofore, in order to decrease such a side lobe level, an antenna unitas shown in FIG. 1 is employed. In FIG. 1, reference characters 1a and1b designate primary radiators, 2a and 2b directional couplers, 3 aresistance attenuator, 4 a phase unit and 5 a main reflecting mirror.The antenna unit is intended to eliminate the side lobe which is formedby an antenna system consisting of the main primary radiator 1a and themain reflecting mirror according to a method in which the primaryradiator 1b is added and a part of the signals from the main feedingsystem, being branched to the directional couplers 2a and 2b, is fed tothe primary radiator 1b with suitable phase and amplitude. That is, aparticular side lobe can be eliminated by providing a beam which isequal in amplitude to and opposite in phase to the side lobe.

However, the method is disadvantageous in that, when it is required toeliminate a number of side lobes, the arrangement is necessarilyintricate as much and accordingly is expensive.

SUMMARY OF THE INVENTION

In view of the above, an object of the present invention is to eliminatethe above described drawbacks accompanying the prior art antenna unit,and the object is accomplished by the provision of an antenna unit inwhich the surface of a reflecting mirror is divided into a plurality ofparts, and the parts are made different in thickness from one another,for instance, by bonding metal tapes suitable in thickness thereto, soas to change the phase distribution on the reflecting surface, wherebythe configuration of the radiation pattern due to the currentdistribution on the reflecting mirror is readily changed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings,

FIG. 1 is an explanatory diagram illustrating the arrangement of aconventional reflecting mirror antenna unit and

FIGS. 2a and 2b illustrate one preferred embodiment of the presentinvention. More specifically, FIG. 2a is a diagram showing the surfaceof a reflecting mirror, which has been divided into plural parts, andFIG. 2b is a diagram showing the parts of the reflecting mirror surface,which are made different in thickness from each other.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention, in which the level of a sidelobe in a particular direction is decreased, will be described withreference to the accompanying drawings.

In the following preferred embodiment, a main reflecting mirror is usedto decrease a radiation field E ( ○H , Φ) in a direction ○H , Φ. In FIG.2b, reference numeral 5 designates a main reflecting mirror andreference numerals 6a, 6b and 6c designate metal tapes different inthickness from one another, which are bonded to the surface of the mainreflecting mirror 5. When the main reflecting mirror 5 is divided into Nparts as shown in FIG. 2a and the radiation field in a direction ○H , Φeach of these parts is represented by E_(n) ( ○H , Φ), then thefollowing expression (1) is obtained. ##EQU1##

Furthermore, e_(n) can be represented by a phase φ_(n) and an amplitude|e_(n) |, that is,

    e.sub.n =|e.sub.n |e.sup.jφ n        (2)

In this connection E₀ is defined as follows, ##EQU2## The data E₀ ismaximum in the direction ○H , Φ, but it scarcely has influences in theother directions. Accordingly, in order that the field in the direction○H , Φ is E₁ as desired and no influence is given in the otherdirections, a value α should be so selected as to meet the followingcondition (4) and accordingly condition (5): ##EQU3## In this case,##EQU4## Therefore, the amplitude and phase of each of the dividedmirror surfaces should be changed by (1+αe^(-j)φ n). However, in thereflecting mirror antenna unit, the amplitude cannot be changed.Accordingly, with respect to the phase only, ##EQU5## Thus, a metal tapehaving a thickness t_(n) corresponding to the phase Φ_(n), should bebonded to each of the divided mirror surfaces as shown in FIG. 2b.

The data t_(n) is defined by the following expression (9): ##EQU6##where k is the wave number, and θ_(n) is the incident angle.

The method has been described with reference to the case where the levelof a side lobe in a particular direction is decreased. However, it canbe readily understood that the method is effectively applied to decreasethe levels of a plurality of side lobes, because the method scarcelygives influence in the other directions.

While the invention has been described with reference to the case wherethe level of a side lobe is decreased, it should be noted that theinvention is not limited thereto or thereby. That is, the currentdistribution on the mirror surface can be changed. Therefore, theinvention is effective in changing the configuration of a beam in a beamantenna unit.

While the embodiment of the invention has been described with referenceto the case where the configuration of a radiation pattern is changed bythe main reflecting mirror, the invention is not limited thereto orthereby. It is obvious that, in the case of a reflecting mirror antennaunit with an auxiliary reflecting mirror, the same effect can beobtained according to a method in which the auxiliary reflecting mirroris divided in N parts, and the radiation field after radio waves fromthese parts have been reflected by the main reflecting mirror arerepresented by e_(n).

What is claimed is:
 1. An antenna unit with a reflecting mirrorcharacterized in that the surface of a reflecting mirror is divided intoa plurality of parts and the parts are made different in thickness byt_(n) from one another, the t_(n) being defined by ##EQU7## where k isthe wave number, θn is the incident angle of a radio wave to saidreflecting mirror and Φ_(n) ' is an amount of phase necessary forvarying a particular radiation pattern, which is defined by ##EQU8## inwhich α is a complex constant having a phase term and an amplitude term,the α being determined as α=(E₁ -E)/E₀ =|α|e^(j)φa so as to make anelectric field in a direction (θ, Φ) at a desired value, in theexpression α, E being a radiation field in a direction (θ, Φ), E₁ beinga desired radiation field in the direction (θ, Φ), E₀ being a radiationfield maximum in the (θ, Φ), φ_(n) being a phase of a radiation fielde_(n) (θ, Φ) in the direction (θ, Φ) of the n-th part of said dividedreflecting mirror and φ_(a) being a phase of said complex constant α. 2.The antenna unit as claimed in claim 1 wherein said plural parts aremade different in thickness from one another by bonding metal tapes onthe surface of said reflecting mirror.